I get the feeling a part of this is testing how to do one big megarealm, which wouldn't be a bad thing.

Debatable. One big mega realm would be cool, but they would probably have to do multiple instances (or whatever) of Org / Stormwind / Shrine / whatever new Expantion Hub City / Starter zones they introduce next expantion to get proper load balancing, otherwise those zones would become a total clusterfuck with 50x the number of people in them then they were really designed for. Tradechat is bad enough on a single high pop realm. Imagine multiplying the spam by a factor of 20.......